1. Field of the Invention
The present invention is directed to a method for time- and location-resolved representation of functional brain activities of a patient, as well as to an arrangement for carrying out the method.
2. Description of the Prior Art
It has been determined that brain activities in the cerebral cortex, caused by stimulation, can be detected by nuclear magnetic resonance tomography. Stimulation experiments of this type have been carried out e.g. with visual stimulation and with stimulation around the primary motor cortex by means of finger movement. Functional brain investigations can also be carried out using other techniques, e.g. PET (positron emission technology) or EEG, however, a considerably better spatial resolution can be achieved by means of nuclear spin tomography.
With real-time data recording, the time resolution is also limited in nuclear magnetic resonance technology due to the limited recording speed, therefore proposals have been made for triggering data acquisition for functional imaging by means of stimulation. For this purpose only a part of the raw data required for a complete image data set is obtained per stimulation. It has been proposed by J. Frahm et al., in SMRM/SMRI Workshop: Functional Imaging of the Brain, Arlington, Jun. 17-19, 1993, to synchronize the data acquisition by periodic repetition of a task that triggers brain activities. A comparable method has already been employed for cine recording of heart movements (see e.g. Dennis Atkinson et al., "Cineangiography of the Heart in a Single Breath Hold with a Segmented TurboFLASH Sequence," in Radiology, 1991, 178, pp. 357-360).
One problem of functional imaging is the Separation of signal changes caused by stimulations, by brain activities, from other signal changes, e.g. caused by movements. In the article by P. A. Bandettini et al., "Processing Strategies for Time-Course Data Sets in Functional MRI of the Human Brain," Magnetic Resonance in Medicine, 30, pp. 161-173 (1993), it was proposed among other things for the solution of this problem to calculate for each pixel a correlation coefficient between the stimulation function and the obtained chronological signal curve. As a stimulation function, periodically repeated stimulations separated by pauses were used. Periodic stimulation functions, however, have several disadvantages:
periodic disturbance processes (e.g. heartbeat, breathing) cannot be separated from the activity signal, and appear as "physiological noise." Processes that show a delay of whole-number multiples of the repetition period likewise cannot be properly recognized. Prolongation of the experiment does not lead to a better suppression of the disturbance in any of these cases. PA1 In addition, periodic stimulation functions have a non-uniform frequency spectrum. Certain spectral components are accordingly excited by the stimulation only weakly or not at all. This introduces a systematic error into the system identification, i.e. the determination of the parameters of a mathematical model.